In telescopic shafts, if the profiled sleeve and the profiled journal are displaced relative to one another under torque, the balls in the inner and outer ball grooves roll substantially without any sliding friction. As a result, the balls and the ball cage with all the balls, together, cover half the relative displacement path between the two elements. The longitudinal displacement unit thus permits a low-friction change in length. At the ends of the displacement path, there are provided axial stops for the ball cage or for the balls, either in the profiled sleeve or on the profiled journal. Under normal operating conditions, the ball cage should preferably not reach the axial stops, but within the displacement region as specified by the design, it should be displaceable without stopping against any elements, thus avoiding any sliding friction of the balls in the ball grooves.
Assemblies of this type are used in torque transmitting shafts, especially in driveshafts. These assemblies, for the purpose of compensating for tolerances of the distance between the attached parts during assembly and/or for compensating for changes in distance of the attached parts, more particularly the joints, require a low-friction adjustment in length under torque loads during operation. The basic design of longitudinal displacement units of this type is known. Under operational conditions a problem occurs in that, if the longitudinal displacement unit is torque-free, vibrations, axial impacts or weight forces can cause the ball cage to move quickly against one of its axial stops. A rolling movement of the balls is not required to that end. If, subsequently, the longitudinal displacement unit is again torque-loaded and if, thereafter, longitudinal compensating movements are required, sliding movements can occur at the balls with the ball cage abutting the end stop. This increases the amount of wear and can lead to sliding plates at the balls as a result of which the function of the longitudinal displacement movement suffers considerably.
U.S. Pat. No. 4,705,491 describes a longitudinal displacement unit for a shaft assembly for torque transmitting purposes of the initially mentioned type which comprises a plurality of rows of balls arranged in a ball cage, wherein the balls of one row of balls consist of an elastic material and are positioned in their ball grooves in a pre-tensioned condition. In this embodiment, the torque capacity of the row of balls is clearly reduced. Thus, there exists a need for an improved longitudinal displacement unit having high torque capacity and improved wear characteristics through reduced sliding movement of the balls under load.